In a bad accident the force that goes through the seat belt is huge, I wouldn't want to attach to something which I don't know the strength of when there is a known good point to attach to right there.

It would help avoid fade, but I just remembered Toyota use 5.1 as standard now, which should be ok for your first track day unless you're Lewis Hamilton.
I did a few track days on Primacy tyres and OEM brakes. Brakes were fine until I did an instructor session, stayed out 20 mins driving hard and they smeared over the discs. If you keep it to 15 minutes you'll likely be fine.

The stock suspension is fairly good, lowering springs are for looks rather than handling. Of you're gonna do anything fit better front brake pads and fluid, otherwise leave it - it will be fine for your first go. Keep it to 15 mins max if you don't upgrade the brakes.

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